High temperature eutectic solder ball attach

ABSTRACT

High melt balls dispersed on the surface of a ball grid array package in such a manner that the collapse of eutectic solder balls is controlled during reflow heating in order to ensure appropriate solder ball standoff, pitch and coplanarity of the ball grid array package from the substrate or printed circuit board surface to which it is being attached.

FIELD OF THE INVENTION

[0001] The present invention pertains generally to the field ofsemiconductors, and more particularly to a method of attachingelectronic components to substrates.

BACKGROUND OF THE INVENTION

[0002] In recent years, flip-chip and ball grid array techniques haveincreasingly been used to connect integrated circuit (IC) chips tointerconnection substrates, such as printed circuit boards and topackage substrates. In flip-chip bonding an IC chip component to ainterconnection substrate or printed circuit board, a plurality (e.g.,an array) of solder balls (also known as “solder bumps”) are formed on asurface of a component, typically the IC chip component, and the bumpedcomponent is brought into a face-to-face relationship with the othercomponent. The two components are then heated (such as in a furnace) toreflow (heat, then allow to cool) the solder bumps between the twocomponents, thereby making electrical connections between respectiveterminals of the two components.

[0003] A need for ever finer pitch arrays of solder balls hasaccompanied an increase in the circuit density of IC chips andmulti-chip modules. For example, an IC chip to be flip-chip connected toan interconnection substrate may require an array of 4 mil diametersolder balls disposed at an 8 mil pitch.

[0004] A “solder ball” refers to a substantially spherical orhemispherical mass or bump of solder (e.g., a lead-tin solder) residenton a substrate (e.g., electronic component), suitable for beingre-flowed to join the electronic component to another electroniccomponent. The term “pitch” refers to the distance between centers ofadjacent solder balls on pads of a substrate. The following units andtheir equivalents are also used herein: 1 mil=0.001 inches; 1micron=0.000001 meters; 25.4 microns=1 mill 1 millimeter=0.001 meters.

[0005] As used herein, a “substrate” is an electronic component having anominally flat surface upon which it is desirable to form solder ballsto effect electrical connections to another electronic component. Ballgrid array (BGA) substrates are substrates. As used herein, the terms“substrate bumping” and “ball bumping” refer to a process for formingsolder balls on substrates.

[0006] The eutectic solder balls on heavy depopulated IC ball grid arraypackages tend to collapse excessively during reflow. This causes theballs to be short and wide, which can cause bridging problems andshorts.

[0007] Accordingly, there exists a need in the industry for a means tolimit or control the collapse of eutectic solder balls during the reflowassembly process.

SUMMARY OF THE INVENTION

[0008] The apparatus comprise high melt balls on an electronic componentsurface to control the collapse of the eutectic solder balls on a BGAduring the reflow process. The high melt balls also ensure coplanarity,consistent device standoff for rework, tighter pitch of solder balls andhigher standoff for reliability.

[0009] A method for controlling the collapse of eutectic solder ballsduring the reflow manufacturing process by providing one or more highmelt solder balls that limit the eutectic solder ball collapse andensure coplanarity of the electronic component surface with thesubstrate surface.

[0010] An electronic assembly comprising an electronic component havinga first surface attached to a first surface of a substrate, wherein thefirst surface of the electronic component is attached to the firstsurface of the substrate according to the following steps: forming anarray of eutectic solder balls across the first surface of theelectronic component; forming more than one high melt ball with apredetermined diameter on the first surface of the electronic component;bringing the eutectic solder balls into contact with the first surfaceof the substrate; and heating the eutectic solder balls without meltingthe high melt balls, such that the eutectic solder balls compress to thestandoff of the diameter of the high melt balls.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] A more complete appreciation of this invention, and many of theattendant advantages thereof, will be readily apparent as the samebecomes better understood by reference to the following detaileddescription when considered in conjunction with the accompanyingdrawings in which like reference symbols indicate the same or similarcomponents, wherein:

[0012]FIG. 1 illustrates a cross-sectional view of a BGA package andsubstrate prior to reflow;

[0013]FIG. 2 illustrates a cross-sectional view of a BGA package andsubstrate after reflow;

[0014]FIG. 3 illustrates a cross-sectional view of a BGA package andsubstrate prior to reflow in accordance with the present invention;

[0015]FIG. 4 illustrates a cross-sectional view of a BGA package andsubstrate after reflow in accordance with the present invention; and

[0016]FIG. 5 illustrates a flow chart for a process to control thecollapse of eutectic solder balls during manufacturing.

DETAILED DESCRIPTION OF THE INVENTION

[0017] As shown in the drawings for purposes of illustration, thepresent invention relates to techniques for providing method forcontrolling the collapse of eutectic solder balls during a reflowheating process.

[0018] Turning now to the drawings, FIG. 1 illustrates a cross-sectionalview of a typical BGA package 100 with eutectic solder balls 110.Eutectic solder balls 110 are generally made of tin/silver/lead, such as62% tin, 2% silver and 36% lead. Other combinations are possible, suchas 63% lead, 37% tin. Most standard solder materials have a meltingtemperature of approximately 175-190 degrees Celsius. Typically, solderballs 110 have a diameter of approximately 15-35 mils prior to thereflow process.

[0019]FIG. 2 shows a cross-sectional view of a typical BGA package 100with eutectic solder balls 230 after the reflow process, in which thesolder balls 110 were brought into contact with a plurality contact pads130 on a substrate, such as a printed circuit board 120 and heatedduring a reflow heating process, in which the eutectic solder ballsliquefy and attached to contact pads 130. During this process, theweight of the BGA package 100 may cause the solder balls 110 to collapsedown to solder balls 230 having a stand-off or height “C” ofapproximately 30-50% of the original solder ball 230 diameter. Solderballs 230 may have a width of approximately 1-2 times the originalsolder ball 230 diameter after the reflow process. This may causebridging or shorting between solder balls 230. This collapse of solderballs 110 may become excessive, especially in the case of heavilydepopulated packages 100.

[0020]FIG. 3 shows a cross-sectional view of a BGA package 300,according to the present invention, with a plurality of eutectic solderballs 310 having an initial diameter “Y” of approximately 25 mils priorto reflow heating and at least one high melt ball 350 having an initialdiameter “X” of approximately 20 mils prior to reflow heating. Eutecticsolder balls 310 may be 62% tin, 2% silver and 36% lead. High melt balls350 may be 90% lead and 10% tin. When a standard reflow heatingtemperature of 175-190 degrees Celsius is used, the high melt balls 350do not reflow, so eutectic paste 340 is used between the high melt balls350 and contact pads 330 to ensure attachment.

[0021] With reference to FIG. 4 a cross-sectional view of the BGApackage 300 attached to substrate 320 after reflow heating is seen.Stretch eutectic solder balls 310 have collapsed to solder balls 360having a standoff height “N” of approximately 20 mils. Post reflowsolder balls 360 have a diameter “M” of approximately 22 mils. Thispermits approximately a 20% increase in spacing between the solderballs. Alternatively the solder ball array pitch may be tightened byapproximately 20%. The technique also has the advantage of higherstandoff, which increases the reliability. It also permits a widerrework process window by ensuring that the package remains parallel tothe printed circuit board on re-attach. One option is to have at leastone high melt ball 350 in each corner of the BGA package 300.Alternatively three or more high melt balls 350 may be located at anyconvenient location(s) on the BGA. Preferably, the high melt balls 350will be spaced in such a manner to ensure coplanarity of BGA 300following the reflow heating process.

[0022] Referring now to FIG. 5 a flow chart illustrates the process ofthe present invention, in which eutectic solder balls 310 are attached510 to the surface of a BGA 300 by any known method. High melt balls 350are attached 520 to the same surface of the BGA 300 by means of eutecticpaste 340 or other known attachment method. The eutectic solder ballsare then brought into contact with fluxed contact pads 330 on asubstrate 320, such as a printed circuit board in a sandwich typefashion. A standard reflow heating process is conducted, such asbringing the sandwich assembly to temperature of approximately 180-210degrees Celsius until the eutectic solder balls 310 reflow and attach tothe contact pads 330 of the substrate 320.

[0023] It will be appreciated from the above detailed description thatthe yield, pitch, and reliability of the BGA solder ball array may beadjusted by adjusting the standoff of the original pre-reflow solderballs, the compound make-up of the stretch eutectic solder balls 310,compound of the high melt balls 350, the reflow heating temperature orreflow process length. Modifying each of these components is fullycontemplated within the scope of the present invention and does notdepart from the general concept of the invention to have one or morehigh melt balls 350 dispersed in a pattern on the surface of a BGApackage 300 to control the collapse of the solder balls 310 during thereflow heating process to permit higher standoff, tighter pitch betweensolder balls 360 and to ensure coplanarity of the BGA package 300 afterthe reflow heating process and if necessary after a rework process.

[0024] Although this preferred embodiment of the present invention hasbeen disclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the invention,resulting in equivalent embodiments that remain within the scope of theappended claims.

What is claimed is:
 1. A ball grid array package, comprising: an array of eutectic solder balls on a surface of the ball grid array package; and at least one high melt ball on the same surface of the BGA package.
 2. The ball grid array package in accordance with claim 1, wherein said at least one high melt ball is at least one high melt ball in each corner of said surface of said ball grid array package.
 3. The ball grid array package in accordance with claim 1, wherein said eutectic solder balls comprise approximately 62% tin, 2% silver, and 36% lead.
 4. The ball grid array package in accordance with claim 1, wherein said at least one high melt ball comprises approximately 90% lead and 10% tin.
 5. The ball grid array package in accordance with claim 1, wherein said at least one high melt ball has a melting point high enough that it does not reflow during a normal reflow heating process to reflow the eutectic solder balls.
 6. The ball grid array package in accordance with claim 1, wherein said eutectic solder balls have a pre-reflow diameter of approximately 20-25 mils.
 7. The ball grid array package in accordance with claim 6 wherein said eutectic solder balls have a post-reflow standoff of approximately 15-20 mils.
 8. The ball grid array package in accordance with claim 7, wherein said at least one high melt ball has a pre-reflow diameter of approximately 20 mils.
 9. An electronic assembly having a first surface, said electronic assembly comprising: an array of eutectic solder balls arranged on said first surface; at least one high melt ball on said first surface of said electronic assembly.
 10. A method for attaching a first surface of an electronic component to a first surface of a substrate, comprising: forming an array of eutectic solder balls on said first surface of said electronic component; attaching at least one high melt ball on said first surface of said electronic component; bringing said array of eutectic solder balls on said first surface of said electronic component into contact with said first surface of said substrate; and heating said array of eutectic solder balls such that said eutectic solder balls reflow and attach to said first surface of said substrate.
 11. The method for attaching a first surface of an electronic component to a first surface of a substrate in accordance with claim 10, wherein said at least one high melt ball does not reflow during said heating step.
 12. The method for attaching a first surface of an electronic component to a first surface of a substrate in accordance with claim 11, wherein said at least one high melt ball comprises a high melt ball in each corner on said first surface of said electronic component.
 13. The method for attaching a first surface of an electronic component to a first surface of a substrate in accordance with claim 12, wherein said first surface of said electronic component is substantially co-planar with said first surface of said substrate following the step of heating the array of eutectic solder balls.
 14. The method for attaching a first surface of an electronic component to a first surface of a substrate in accordance with claim 10, wherein said eutectic solder balls are comprised substantially of 62% tin, 2% silver, and 36% lead.
 15. The method for attaching a first surface of an electronic component to a first surface of a substrate in accordance with claim 14, wherein said at least one high melt ball is comprised substantially of 90% lead and 10% tin.
 16. The method for attaching a first surface of an electronic component to a first surface of a substrate in accordance with claim 15, wherein said heating step comprises heating said array of eutectic solder balls to approximately 175-190 degrees Celsius.
 17. The method for attaching a first surface of an electronic component to a first surface of a substrate in accordance with claim 10, wherein each eutectic solder ball in said array of eutectic solder balls has an initial diameter of approximately 20-25 mils.
 18. The method for attaching a first surface of an electronic component to a first surface of a substrate in accordance with claim 17, wherein said at least one high melt ball has an initial diameter of approximately 20 mils.
 19. The method for attaching a first surface of an electronic component to a first surface of a substrate in accordance with claim 18, wherein after the heating step, said array of eutectic solder balls has a standoff of approximately 15-20 mils.
 20. The method for attaching a first surface of an electronic component to a first surface of a substrate in accordance with claim 19, wherein after the heating step, each eutectic solder ball in said array of eutectic solder balls has a diameter of approximately 22 mils.
 21. An electronic assembly comprising an electronic component having a first surface attached to a first surface of a substrate, wherein the first surface of the electronic component is attached to the first surface of the substrate according to the following steps: forming an array of eutectic solder balls across the first surface of the electronic component; forming more than one high melt ball with a predetermined diameter on the first surface of the electronic component; bringing the eutectic solder balls into contact with the first surface of the substrate; and heating the eutectic solder balls without melting the high melt balls, such that the eutectic solder balls compress to the standoff of the diameter of the high melt balls. 